1. Field of the Invention
The present invention relates to a display control device for an image display system, and more particularly to a display control device for use in an image display system having an image memory, for instance, a videotex system such as the CAPTAIN or NAPLPS system, which stores transmitted image data in the image memory and reads the image data out of the image memory in sequence to display it on a display device.
2. Description of the Related Art
In a videotex system adapted for information delivery services using telephone lines, a cathode ray tube (CRT) of a television receiver is generally used as a display device for displaying transmitted information visually.
In recent years liquid crystal display devices (LCDs) have widely been used as display devices for electronic equipment. The LCDs are classified into active-matrix type LCDs used with liquid crystal television receivers and binary image type LCDs used with word processors and personal computers. The binary image type LCDs have, for example, 640.times.400 pixels each having two display states of on and off.
A liquid crystal television receiver using the active-matrix type LCDs can display images in colors and gradations like CRT display devices and may be used as display devices for the videotex because of provision of video interface. On the other hand, the binary image type LCDs are not currently in use as display devices for videotex. The reasons therefor may include the following two:
(1) Image data used in the videotex system is composed of dot pattern data and color data and a frame is composed of two frames of a code frame and a photo frame. To display images on the binary image type LCDs by using the image data for the videotex, predetermined data conversion processes would be required.
(2) The CRT and the binary image type LCD require different driving methods.
In view of the fact that equipment using the binary image type LCDs is widely used, the use of the binary image type LCDs as display devices for the videotex system should be considered.
However, in using the binary type LCDs as display devices for the videotex the following two problems must be taken into consideration.
A first problem may arise in constructing a videotex system which can use both active matrix type LCDs (or CRTs) and binary type LCDs. That is, if a display control device capable of controlling such different types of display devices simultaneously were constructed, a separate display control section would generally be provided for each of the display devices. More specifically, a display control section for an active matrix type LCD (or CRT) and a display control section for a binary type LCD would separately be connected to a central processing unit (CPU) for performing data processing such as data conversion on image data transmitted from an information center.
With such a construction, however, software used with the CPU would become complex because the CPU would have to carry out different data conversion for two types of display devices. In hardware as well, the provision-of two separate display control sections would increase circuit scale.
Next, a second problem may arise from the fact that the frames in the videotex system comprise two types of frame, one for code and the other for photo as described above, while the frames that the binary type LCDs handle comprise only one type of frame, and moreover the binary type LCDs can accommodate only dot pattern data indicative of brightness. That is, when dot pattern data of both of a code frame and a photo frame are displayed one above the other on a binary type LCD, they may not be distinguished from each other. This problem will arise both in the case where a videotex system that can accommodate the active matrix type LCDs (or CRTs) and the binary type LCDs is constructed and in the case where videotex system that can accommodate only the binary type LCDs is constructed.
As described above, in the art of videotex systems, the development of a display control device is desired which is adaptable to two types of display devices, such as an active matrix type LCD (or CRT) and a binary type LCD, which are different from each other in image data structure and driving method. In this case there arise problems of complexity of software used and an increase in amount of hardware used. In addition, there is a problem of how to discriminate between a code frame and a photo frame in displaying image data of videotex on a binary type LCD.